The authors of the study funded by the US National Science Foundation stressed that global warming is real, and that increases in atmospheric CO2, which has doubled from pre-industrial standards, will have multiple serious impacts.

not taking into account a fully integrated paleoclimate date on a global scale.

The researchers based their study on ice age land and ocean surface temperature obtained by examining

ices cores, bore holes, seafloor sediments and other factors.

When they first looked at the paleoclimatic data, the researchers only found very small differences in ocean temperatures then compared to now.

"Yet the planet was completely different -- huge ice sheets over North America and northern Europe, more sea ice and snow, different vegetation, lower sea levels and more dust in the air," said Schmittner.

From these models, scientists find it difficult to narrow their projections down to a single figure with any certainty, and instead project a range of temperatures that they expect, given a doubling of atmospheric CO2 from pre-industrial levels.

The new analysis, which incorporates palaeoclimate data into existing models, attempts to project future temperatures with a little more certainty.

Lead author Andreas Schmittner from Oregon State University, US, explained that by looking at surface temperatures during the last Ice Age - 21,000 years ago - when humans were having no impact on global temperatures, he, and his colleagues,